(I've also discovered the PS3 youtube app which can sync with my phone to be pretty nifty too)
It's in the PS3 store in the Apps section. Some people have said they also found it in the TV / Videos services of the main menu on the PS3. They kinda just snuck it in there without any fanfare.It uses the web browser on the phone so works on iPhones, Android and others without having to download any apps on them.
Thanks for sharing, I'm going to test that out later today.